1. Field of the Invention
The present invention relates to an optical head apparatus for different types of disks such as a digital versatile disk (DVD) and a compact disk (CD).
2. Description of the Related Art
Recently, optical head apparatuses have been developed to be adapted to a DVD and a CD. In this case, a DVD requires a 650 nm wavelength light source, while a CD requires a 780 nm wavelength light source. Therefore, one of such optical head apparatuses is constructed by two light sources for emitting two kinds of light beams, i.e., a 650 nm wavelength light beam and a 780 nm wavelength light beam and transmitting them to a disk and one photodetector for receiving a light beam reflected from the disk.
In a first prior art optical head apparatus (see: JP-A-10-112050), first and second beam splitters for receiving a 650 nm wavelength light beam and a 780 nm wavelength light beam, respectively, are provided between a photodetector and an objective lens. The first beam splitter transmits half of the 650 nm wavelength light beam and most of the 780 nm wavelength light beam therethrough, while the first beam splitter reflects half of the 650 nm wavelength light beam and a small part of the 780 nm wavelength light beam. Also, a second beam splitter transmits most of the 650 nm wavelength light beam and half of the 780 nm wavelength light beam, while the second beam splitter reflects a small part of the 650 nm wavelength light beam and half of the 780 nm wavelength light beam. This will be explained later in detail.
In the above-described first prior art optical head apparatus, however, since losses of the intensity of the outgoing and incoming 650 nm wavelength light beams occur in the first beam splitter and also, losses of the intensity of the outgoing and incoming 780 nm wavelength light beams occur in the second beam splitter, the signal-to-noise (S/N) ratio would be decreased.
In a second prior art optical head apparatus (see: JP-A-10-228668), a wavelength selecting prism for selecting an S-polarized 650 nm wavelength light beam and an S-polarized 780 nm light beam is provided, and a beam splitter and a quarter-wave plate are provided between a photodetector and an objective lens. The beam splitter transmits most of the P-polarized components of the 650 nm wavelength light beam and the 780 nm wavelength light beam, while the beam splitter reflects most of the S-polarized components of the 650 nm wavelength light beam and the 780 nm wavelength light beam. This also will be explained later in detail.
In the above-described second prior art optical head apparatus, however, since losses of the intensity of the outgoing light beams occur in the wavelength selecting prism, the S/N ratio would be decreased. Additionally, since the difference in wavelength dependence characteristics between the P-polarized and S-polarized light beams cannot be large, it is difficult for the 650 nm wavelength light beam and the 780 nm wavelength light beam to be within a range of the polarizing beam splitter characteristics. Therefore, since the manufacturing margin of the beam splitter is small, the manufacturing cost would be increased.